Impellers are generally known and typically include a hub section, for example, in a form of a disc mounted on a rotatable shaft, and a plurality of blades attached to the hub section. A shroud or side wall is connected to the hub section and the plurality of blades. The shroud may be connected to the hub section and the plurality of blades only on one side of the plurality of blades, and such an impeller is referred to as a semi-open impeller. Alternatively, the shroud may be connected to the hub section and the plurality of blades on both sides of the plurality of blades and, as such, encloses the plurality of blades. Such an impeller is referred to as an enclosed impeller. In some impellers, the shroud may be absent and such impellers are referred to as open impellers. In operation, fluid enters an impeller through an inlet opening located proximal to the shaft, flows radially outwardly through flow channels defined between the plurality of blades, and exits the impeller through one or more outlets located at the outer perimeter of the impeller.
During the hardening process step in the manufacturing of an impeller, the impeller may be heated to a required temperature and quenched in air, gas, and/or liquid. Often, the impeller is large in size and has a relatively thick hub section. The large size and the thickness of the hub section of the impeller may affect the heating and cooling rates of the material of the hub section. For example, when exposed to quenching material (air, gas and/or liquid), the material in a section of the impeller having a larger cross-sectional thickness (for example, material in and around a central opening of the hub section) may not cool as rapidly as compared to the material on the surface of the impeller and the material in a section of the impeller having a smaller cross-sectional thickness (for example, material on the periphery of the impeller). As a result, the material in and around the central opening of the hub section of the impeller may be softer than the material on the surface, and material in and around the periphery of the impeller. There may be, therefore, a difference in the microstructure of impeller material throughout the entire structure of the impeller. Consequently, the impeller may exhibit less than desirable mechanical properties, for example, reduced hardness, reduced compressive strength, reduced yield strength, or the like, in the hub section. This may result in deformities in the hub section of the impeller during operation. For example, the impeller may experience yielding during overspeed.
As a result, presently known methods utilized in an effort to improve the mechanical properties in the thick hub section include using new manufacturing methods which are not viable for larger impellers, using expensive materials which erode profit margin, using various types of heat treatments that provide less than the desired mechanical properties, and designing hub sections of different geometrical configurations.
What is needed, therefore, is a method of achieving the desired mechanical properties at the thick hub section without substituting the standard material or altering the present manufacturing processes.